The present invention relates to a microwave input circuit with parametric upper sideband down converter which is directly connected to a microwave antenna. More particularly, the present invention relates to such a parametric down converter which is terminated by a real impedance at the input signal frequency which is not much different from the intermediate frequency produced at the output of the down converter.
Parametric amplifiers per se are known, for example from the book, VARACTOR APPLICATIONS, by P. Penfield and R. P. Rafuse, published by M.I.T. Press, 1962. It is also known from U.S. Pat. No. 3,991,373, issued Nov. 9, 1976 to Maurer et al. (the subject matter of which is incorporated herein by reference) to directly connect a down converter to the receiving antenna for microwaves and to employ for this purpose a parametric upper sideband down converter which is terminated with a real impedance at the image frequency. However, the down converter disclosed in this patent will have little noise only if the input signal frequency f.sub.s1 is much greater than the intermediate frequency f.sub.z produced at the output of the down converter (see column 4, equation (5) of the patent).